Robust cable-type detection for usb power delivery

ABSTRACT

A system and method for detecting a USB cable-type. A USB PD device configured at a near end of a USB cable is configured to (i) receive and process a signal from a device at a far end of the USB cable to determine a power rating of the USB cable and (ii) adjustably establish power delivered by the first device to the USB cable as a function of the determined USB cable power rating.

CLAIM OF PRIORITY

This application is a continuation of U.S. Nonprovisional patentapplication Ser. No. 13/969,170, filed Aug. 16, 2013 (Now U.S. Pat. No.______), which claims priority of U.S. Provisional Patent ApplicationSer. No. 61/738,937 Entitled ROBUST CABLE-TYPE DETECTION FOR USB POWERDELIVERY, filed Dec. 18, 2012, the contents of which are incorporatedherein by reference in their entirety.

TECHNICAL FIELD

This disclosure is generally directed to Universal Serial Bus (USB)cables and more particularly to USB power-delivery (PD) cables.

BACKGROUND

A new USB power delivery (PD) specification is under development toenable delivery of higher power over new USB cables and connectors. Theintent for this technology is to create a universal power plug forlaptops, tablets, and other devices that may require more than 5V. TheUSB PD specification defines a communication link between portsconnected via a USB-PD cable and connectors. The communication isdesigned to be half-duplex and packet-based. The packets contain variousinformation that enables the two ports to communicate and negotiate,including the voltage and the current that the source port will provideto the sink port. The ports can even negotiate to switch roles (Sourceto Sink and vice versa). The underlying communication in the USB PDspecification is binary frequency shift keying (FSK). This communicationhappens independently from normal USB communications that go through thesame cable but over different wires. The USB PD communication goes overthe “Vbus” wire rather than the USB data wires.

Since legacy cables and even new PD cables have different currentratings, one piece of the new specification is the ability to detect thecable-type and thereby know the current rating of a cable. USB PDdevices are not allowed to request or offer a voltage or currentexceeding the ratings of the cable. Legacy cables (standard B, standardA, micro-A or micro-B) are limited to 1.5 A and 5V. PD micro cables(type A and B) are limited to 3 A. PD standard cables are limited to 5A.

In an equivalent circuit for transmission in a USB PD system, ideallyrTX=Z0 so that there are not any reflections from the cable duringtransmission. In some cases, the receiver may also use a value of rRXapproximately equal to Z0 so that there are not reflections back ontothe cable. In other cases, the receiver may have rRX set to a value muchgreater than Z0, in which case the line is effectively not terminated.In the latter case, if the cable length is approximately a quarter of awavelength, the voltage seen at the input to the cable on the TX side isclose to zero. This creates some challenges for a conventional USB PDsystem.

SUMMARY

Embodiments of this disclosure include a system and method for detectinga USB cable-type. A USB PD device configured at a near end of a USBcable is configured to (i) receive and process a signal from a device ata far end of the USB cable to determine a power rating of the USB cableand (ii) adjustably establish power delivered by the first device to theUSB cable as a function of the determined USB cable power rating.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this disclosure and its features,reference is now made to the following description, taken in conjunctionwith the accompanying drawings, in which:

FIG. 1 illustrates how USB PD devices may be deployed, where any givendevice could be a Provider/Consumer (P/C), Consumer/Provider (C/P),Provider-only, or Consumer-only;

FIG. 2 illustrates a circuit of how two ports may be connected;

FIG. 3 illustrates a packet structure for a typical packet;

FIG. 4 illustrates a hard reset packet structure;

FIG. 5 illustrates a bitstream signal structure of a preamble;

FIG. 6 illustrates an equivalent circuit for a transmitter and receiver;

FIG. 7 illustrates a cable-type detection circuit for non-Standard-Areceptacles;

FIGS. 8 and 9 illustrate methods of determining plug type;

FIG. 10 illustrates a system block diagram of an example USB PD deviceaccording to this disclosure;

FIGS. 11A-11C illustrate schematics of A plug connector markers;

FIGS. 12A and 12B illustrate schematics of B plug connector markers; and

FIG. 13 illustrates an example method according to this disclosure.

DETAILED DESCRIPTION

FIGS. 1 through 13, discussed below, and the various embodiments used todescribe the principles of the present invention in this patent documentare by way of illustration only and should not be construed in any wayto limit the scope of the invention. Those skilled in the art willunderstand that the principles of the present invention may beimplemented in any suitable manner and in any type of suitably arrangeddevice or system.

FIG. 1 illustrates how USB PD devices may be deployed, where any givendevice could be a Provider/Consumer (P/C), Consumer/Provider (C/P),Provider-only, or Consumer-only. FIG. 2 illustrates a circuit of how twoports may be connected. FIG. 3 illustrates a packet structure for atypical packet. A preamble is followed by a start of packet (SOP), aheader, a payload (if any), a cyclic redundancy check (CRC) (such as a32-bit value) covering the header and payload, and an end of packet(EOP). Control messages are packets that do not have any payload. Thisdisclosure mentions some of the specific kinds of control messages, andthe details of their definition are provided herein for clarity. Table 1shows how the header is defined.

TABLE 1 Message Header Bits(s) Field Name Notes 15 Reserved Shall be setto 0 14 . . . 12 Number of Data Objects 11 . . . 9  MessageID See Table2  8 Port Role 7 . . . 6 Specification Revision 5 . . . 4 Reserved Shallbe set to 0 3 . . . 0 Message Type

For Control messages, the Number of Data Objects is set to 000. Thedefinition of the Message Type field is shown in Table 2. The othernon-reserved fields may have any value. There are 13 different controlmessages shown in Table 2.

TABLE 2 Control Message Types Bits 3 . . . 0 Message Type Sent byDescription 0000 Reserved N/A All values not explicitly defined arereserved and shall not be used. 0001 GoodCRC Source or Sink 0010 GotoMinSource only 0011 Accept Source or Sink 0100 Reject Source or Sink 0101Ping Source only 0110 PS_RDY Source only 0111 Get_Source_Cap Source orSink 1000 Get_Sink_Cap Source or Sink 1001 Protocol Error Source or Sink1010 Swap Source or Sink 1011 Reserved Sink only 1100 Wait Source only1101 Soft Reset Source or Sink 1110- Reserved N/A All values notexplicitly defined 1111 are reserved and shall not be used.

FIG. 4 illustrates a hard reset packet structure. FIG. 5 illustrates abitstream signal structure of a preamble.

In USB PD terminology, a device's capabilities include the amount ofcurrent and voltage it can source and/or sink. A device may not bepermitted to offer or request current exceeding the capability of thecable. That is why cable-type detection is such an important part of theUSB PD system.

FIG. 6 illustrates an equivalent circuit for a transmitter and receiver.Ideally, rTX=Z0 so that there are not any reflections from the cableduring transmission. In some cases, the receiver may also use a value ofrRX approximately equal to Z0 so that there are not reflections backonto the cable. In other cases, the receiver may have rRX set to a valuemuch greater than Z0, in which case the line is effectively notterminated. In the latter case, if the cable length is approximately aquarter of a wavelength, the voltage seen at the input to the cable onthe transmitter (TX) side is close to zero. This creates some challengesfor a conventional USB PD system.

FIG. 7 illustrates a cable-type detection circuit 10 for non-Standard-Areceptacles. The circuit 10 shown in FIG. 7 may be used to detect theelectronic markings on an ID pin indicating the type of micro-connectorof a cable. FIGS. 8 and 9 illustrate methods of determining plug type.The transmitter (TX) is used to put a carrier signal on the “Vbus” lineon the near end of the cable, and the receiver (RX) is used to detectwhether a signal is present or not at the near end of the cable(typically a Squelch receiver is used for this purpose). Switches Q1-Q4are used to create a series of circuits where the voltage output (asmeasured by the RX) in each step is used to determine the configurationof the plug in accordance with Table 3 below. In normal operation,switch Q1 is conducting (turned ON), and switches Q2-Q4 are notconducting (turned OFF). In order to check the plug type using thecircuit in FIG. 8, a series of steps can be performed, and the result ofeach step can be recorded as a “0” or “1.” The steps could include:

-   -   1) Transistors Q1, Q3, and Q4 are not conducting (turned OFF).    -   2) Transistor Q2 is conducting (turned ON).    -   3) Check Squelch->ope—“1”, else “0”->bit 1.    -   4) Transistor Q3 is conducting (turned ON).    -   5) Check Squelch->open—“1”, else “0”->bit 2.    -   6) Transistor Q4 is conducting (turned ON).    -   7) Check Squelch->open—“1”, else “0”->bit 3.        Table 3 below summarizes the results.

In Step 3, if Vbus is short-circuited (due to an unterminated cablewhose length is a quarter of a wavelength, such as roughly 2.5 m), theundesirable result of the cable-type detection is Fault, which may ormay not be the case. This means that, using this circuit and algorithm,the cable-type cannot be determined. Other circuits may be able todetect some cable types using a DC voltage that is not connected toVbus, for example, to detect a resistor between the ID and GND pins.FIGS. 11A-11C illustrate schematics of A plug connector markers, whichoperate in this manner. Other circuits would not be able to solve thisproblem completely since, in some cases, the voltage on Vbus needs to beconsidered to detect if there is a capacitor between it and the ID pin.FIGS. 12A and 12B illustrate schematics of B plug connector markers,which operate in this manner.

The squelch receiver referred to above is a circuit that detects ifthere is a signal on the line with a large amplitude. Typically, itshould trigger (be open) when the signal level exceeds 20 mVrms. Thefunctionality of the squelch receiver may sometimes be replaced by anFSK receiver, which will be “open” if it can demodulate bits reliably.

TABLE 3 Plug Type Determination Micro-B or Approx. level Approx. levelApprox. level Micro-A PD Standard- at RX when at RX when at RX when bit1 bit 2 bit 3 plug B plug detecting bit 1 detecting bit 2 detecting bit3 1 1 1 Low PD (5A) ~0 dB ~0 dB ~−9 dB Power 1 1 0 PD Legacy ~0 dB ~−6dB (PD) ~−30 dB ~0 dB (Legacy) 1 0 1 Fault Fault 1 0 0 Legacy PD (3A) ~0dB ~−40 dB ~−40 dB 0 1 1 Fault Fault 0 1 0 Fault Fault 0 0 1 Fault Fault0 0 0 Fault Fault

In accordance with this disclosure, the disclosed cable-type detectionmechanism and method advantageously use and process a packet transmittedby a far-end of a USB PD cable to detect the presence of capacitors inone or both plugs of the cable. This detection mechanism is coupled to aUSB receptacle of a USB PD device and may reside in the USB PD device,although other configurations are envisioned. The detection mechanism isdescribed in the context of the cable-type detection circuit 10 in FIG.7 in view of FIG. 10, FIGS. 11A-11-C, and FIGS. 12A-12B. This mechanismcan be extended to other circuits, as well.

In some embodiments, the detection mechanism resides in a near end USBPD device and is coupled to a receptacle that receives a near end of aUSB PD cable. The USB PD device is configured to receive and process acharacteristic signal, such as an AC signal or packet data, sent by afar-end device of the cable on Vbus (instead of the near-end devicegenerating its own AC signal). This overcomes the problem when thenear-end device always sees close to 0V on Vbus when it sends an ACsignal on Vbus (due to an unterminated cable as previously described).The USB PD device is configured to deliver power to the far end device44 while simultaneously detecting the presence of the characteristicsignal from the far end device 44.

FIG. 10 illustrates a system block diagram of an example USB PD deviceaccording to this disclosure, and FIG. 13 illustrates an example method50 according to this disclosure. A USB PD controller 20 is configured toreside in or otherwise be associated with a USB PD device 22. The USB PDcontroller 20 is coupled to a USB receptacle 24, which is configured toreceive a near end 25 of a cable 26. Other applications may also use theUSB PD Controller 20 and the cable-type detection mechanism outlinedbelow.

In FIG. 10, Q is a vector representing the values of the switch statesof the transistors Q1-Q4 shown in FIG. 7 and previously discussed. Amultipoint control unit (MCU) or other logic device 28 controls acable-type detection circuit 30 through Q. A TX block 32 in an FSK modem34 may or may not terminate a line 36 coming from the cable-typedetection circuit 30. A RX block 38 and/or a Squelch RX block 40terminates the line from the USB receptacle 24 with a large resistanceor impedance. The RX block 38 can pass demodulated bits to the logicdevice 28, although it may deliver only the header and payload uncodedbits along with the packet type. The logic device 28 carries outprotocol, policy engine, and device policy manager functions. The policyengine interprets the policy manger's input in order to implement policyfor a given port and directs the protocol layer to send appropriatemessages. The device policy manager applies local policy to each portvia the policy engine. In some embodiments, the logic device 28 may alsocarry out other functions, such as CRC detection, 4b5b decoding, orother functions.

The exact division of tasks between the logic device 28 and the RX block38 can vary depending on the design parameters. The logic device 28 isconfigured to send bits it wants to transmit to the TX block 32. The TXblock 32 may add 4b5b encoding, a preamble, SOP, CRC, and/or EOP, andthe logic device 28 is not necessarily required to construct the exactpacket. Based on the characteristic signal generated by a far end device42 at a far-end 44 of the cable 26, the logic device 28 is configured toprocess the received signals and responsively send signals to externalpower switches S1-S3 to control parameters of the Vbus line connected tothe power system of the device. For example, it may sink 5V or source20V.

The USB PD device 22 connected to receptacle 36 can assume a legacycable 21 is attached until it knows otherwise, as this protects it fromoffering or requesting current from a far-end device that may exceed thecable capability. The USB PD device 22 can execute the cable-typedetection procedure described above. If the result is a “Fault” becausebit 1=0, the procedure described below can then be executed. Otherwise,the cable-type is already known using the cable-type detection proceduredescribed above.

Standard-B Receptacle

A USB PD device 22 with a Standard-B receptacle is a C/P or C-onlydevice. According to the USB-PD system, the first data packetcharacteristic signal that the device 22 receives from the far-enddevice 44 of the cable 26 is more than likely either a Capabilitiesmessage, a HardReset packet, or a BitStream. It is also allowed to senda Get_source_cap message and a SoftReset message, in which case it mayreceive a GoodCRC message.

If the cable-type is unknown, the USB PD device 22 with a standard-Breceptacle 24 may know that the cable 26 is either PD (5 A), PD (3 A),or a legacy cable. The USB PD device 22 can wait until it receives adata packet from the far-end device 44 of the cable 26 to determine thecable type. When the far-end device 44 on the cable 26 sends a datapacket, the Vbus wire does not appear to the USB PD device 22 to beshorted any more. While waiting for the first packet, transistor Q1 isclosed while transistors Q2-Q4 are open. The following steps can befollowed according to one embodiment to determine cable type:

-   -   1. When the squelch RX 40 and/or FSK modem 34 detects a data        packet arriving, open transistor Q1 and close transistors Q2 and        Q3.    -   2. If the squelch RX 40 and/or FSK modem 34 still detects a        packet, there is a capacitor coupling the Vbus and ID pins as        shown in FIG. 12B. The cable type is PD (5 A). Close transistor        Q1, and open transistors Q2-Q4. Exit the procedure.    -   3. Else, close transistor Q1 (transistors Q2 and Q3 remain        closed).    -   4. If the squelch RX 40 and/or FSK modem 34 detects a packet,        there is not a capacitor coupling the ID and GND pins. The cable        type is Legacy as shown in FIG. 11A. Close transistor Q1, and        open transistors Q2-Q4. Exit the procedure.    -   5. Else, open transistors Q2 and Q3 (transistor Q1 remains        closed).    -   6. If the squelch RX40 and/or FSK modem 34 detects a packet,        there is a capacitor coupling the ID and GND pins as shown in        FIG. 12A. The cable type is PD (3 A). Close transistor Q1, and        open transistors Q2-Q4. Exit the procedure.    -   7. Else, the cable type is still unknown. Close transistor Q1,        and open transistors Q2-Q4. Exit the procedure.

Note that following this procedure may cause the packet CRC to fail. Theprotocol handles this failure and causes more packets to be sent. Aftersome attempts, if the cable-type is still undetermined, the device maychoose to assume it is a legacy cable. Alternatively, to avoid causingissues in the policy engine the device may choose to perform thecable-type detection mechanism on a subset of the arriving packets. Thisensures that packets that are retried will be processed as the policyengine expects avoiding hard resets that would disrupt the normal USB PDoperation. For example, only every other packet could be used.

Micro-B Receptacle

A USB PD device 22 with a Micro-B receptacle 24 is a C/P or C-onlydevice. According to the USB-PD system, the first data packet that thedevice 22 receives from the far-end device 44 of the cable 26 is morethan likely either a Capabilities message, a HardReset packet, or aBitStream. It is also allowed to send a Get_source_cap message and aSoftReset message, in which case it may receive a GoodCRC message.

If the cable-type is unknown, the USB PD device 22 with a Micro-Breceptacle 24 may know that the cable 26 is either PD (3 A) as shown inFIG. 12A or a legacy cable as shown in FIG. 11A. The USB PD device 22can wait until it receives a data packet from the far-end device 44 onthe cable 26 to determine the cable type. When the far-end device 44 onthe cable 26 sends a data packet, the Vbus wire does not appear to theUSB PD device 22 to be shorted any more. While waiting for the firstpacket, transistor Q1 is closed while transistors Q2-Q4 are open. Thefollowing steps can be followed according to one embodiment to determinecable type:

-   -   1. When the squelch RX 40 and/or FSK modem 34 detects a data        packet arriving, close transistors Q2 and Q3 (transistor Q1        remains closed).    -   2. If the squelch RX 40 and/or FSK modem 34 detects a packet,        there is not a capacitor coupling the ID and GND pins as shown        in FIG. 11A. The cable type is Legacy. Close transistor Q1, and        open transistors Q2-Q4. Exit the procedure.    -   3. Else, open transistors Q2 and Q3 (transistor Q1 remains        closed).    -   4. If the squelch RX 40 and/or FSK modem 34 detects a packet,        there is a capacitor coupling the ID and GND pins as shown in        FIG. 12A. The cable type is PD (3 A). Close transistor Q1, and        open transistors Q2-Q4. Exit the procedure.    -   5. Else, the cable type is still unknown. Close transistor Q1,        and open transistors Q2-Q4. Exit the procedure.

Note that following this procedure may cause the packet CRC to fail. Theprotocol handles this failure and causes more packets to be sent. Aftersome attempts, if the cable-type is still undetermined, the device maychoose to assume it is a legacy cable. Alternatively, to avoid causingissues in the policy engine the device may choose to perform thecable-type detection mechanism on a subset of the arriving packets. Thisensures that packets that are retried will be processed as the policyengine expects avoiding hard resets that would disrupt the normal USB PDoperation. For example, only every other packet could be used.

Micro-AB Receptacle

A USB PD device 22 with a Micro-AB receptacle 24 could be any type ofUSB PD device (C/P, C-only, P/C, P-only). According to the USB-PDsystem, the first data packet that the device 22 receives from thefar-end device 44 of the cable 26 is more than likely either aCapabilities message, a HardReset packet, a BitStream, or aGet_source_cap message. It may also send a Get_source_cap message,SoftReset message, or a capabilities message, in which case it mayreceive a GoodCRC message.

In this situation, the possible cable-types cannot be narrowed down fromthe initial cable-type detection attempt. As a result, the entireprocedure can be performed but in a different order as shown below.Determining the value of bit1 last is necessary because the device doesnot control the duration of the arriving packet. Therefore, the laststep checks that the packet is still arriving which means the values ofbit2 and bit3 were determined while there was a signal present.

-   -   1. When the squelch RX 40 and/or FSK modem 34 detects a packet        arriving, close transistors Q2 and Q3 and open transistor Q1        (transistor Q4 remains open).    -   2. Check Squelch→if open, set bit2 to “1”, else set bit2 to “0”    -   3. Close transistor Q4 (transistor Q1 remains open).    -   4. Check squelch→if open, set bit3 to “1”, else set bit3 to “0”    -   5. Close transistor Q1, and open transistors Q2-Q4.    -   6. Check squelch→if open, set bit1 to “1”, else set bit1 to “0”

The cable-type is again determined using the values in bit1, bit2, andbit3 along with Table 3. If the cable-type is still unknown, the USB PDdevice 22 can wait for the next data packet and try again. After someattempts, if the cable-type is still undetermined, the device 22 maychoose to assume it is a legacy cable. Note that following thisprocedure may cause the packet CRC to fail. The protocol handles thisfailure and cause more packets to be sent. Alternatively, to avoidcausing issues in the policy engine the device may choose to perform thecable-type detection mechanism on a subset of the arriving packets. Thisensures that packets that are retried will be processed as the policyengine expects avoiding hard resets that would disrupt the normal USB PDoperation. For example, only every other packet could be used.

According to the procedures of the embodiments outlined above, the USBPD device may complete cable-type detection only after receiving apacket from the far-end of the cable. For devices that are sources, thismeans that the capabilities they offer may change after receiving apacket from the far end of the cable since they may increase the currentthey offer beyond 1.5 A (the legacy cable limit). For devices that aresinks, this means that the current they request may exceed 1.5 A (thelegacy cable limit). The procedures described above could be modified towork with other existing or future cable-type detection circuits.

All of these approaches are generally illustrated in FIG. 13, where themethod 50 includes a device at the far end of a USB cable sending asignal over the cable (step 52) and a USB PD device at the near end ofthe cable receiving the signal (step 54). The method 50 also includesthe USB PD device determining the cable-type of the USB cable based onthe received signal (step 56). At that point, the USB PD deviceestablishes a sink or source current or other aspect of the cable's useas a function of the determined cable-type (step 58).

Although the above description has described specific embodiments of acable-type detection mechanism, various changes may be made to thedetection mechanism. For example, the detection mechanism is not limitedto use with the circuit of FIG. 7 or FIG. 10. Moreover, the detectionmechanism has been described as assuming a limited number of potentialUSB cable types, such as specific receptacles (standard-B, micro-B, andmicro-AB) and currents (1.5 A, 3 A, and 5 A). However, the detectionmechanism described above can be modified to support the use of anysuitable receptacles (now existing or developed later) and currents,voltages, or other operating characteristic(s).

In some embodiments, various functions described above are implementedor supported by a computer program that is formed from computer readableprogram code and that is embodied in a computer readable medium. Thephrase “computer readable program code” includes any type of computercode, including source code, object code, and executable code. Thephrase “computer readable medium” includes any type of medium capable ofbeing accessed by a computer, such as read only memory (ROM), randomaccess memory (RAM), a hard disk drive, a compact disc (CD), a digitalvideo disc (DVD), or any other type of memory. A “non-transitory”computer readable medium excludes wired, wireless, optical, or othercommunication links that transport transitory electrical or othersignals. A non-transitory computer readable medium includes media wheredata can be permanently stored and media where data can be stored andlater overwritten, such as a rewritable optical disc or an erasablememory device.

It may be advantageous to set forth definitions of certain words andphrases used throughout this patent document. The term “couple” and itsderivatives refer to any direct or indirect communication between two ormore elements, whether or not those elements are in physical contactwith one another. The terms “transmit,” “receive,” and “communicate,” aswell as derivatives thereof, encompass both direct and indirectcommunication. The terms “include” and “comprise,” as well asderivatives thereof, mean inclusion without limitation. The term “or” isinclusive, meaning and/or. The phrase “associated with,” as well asderivatives thereof, may mean to include, be included within,interconnect with, contain, be contained within, connect to or with,couple to or with, be communicable with, cooperate with, interleave,juxtapose, be proximate to, be bound to or with, have, have a propertyof, have a relationship to or with, or the like.

While this disclosure has described certain embodiments and generallyassociated methods, alterations and permutations of these embodimentsand methods will be apparent to those skilled in the art. Accordingly,the above description of example embodiments does not define orconstrain this disclosure. Other changes, substitutions, and alterationsare also possible without departing from the spirit and scope of thisdisclosure, as defined by the following claims.

What is claimed is:
 1. A system comprising: a universal serial bus (USB)cable having a near end and a far end and including a Vbus wire; a firstdevice having a receptacle configured to be coupled to the near end ofthe USB cable; and a second device configured to be coupled to the farend of the USB cable, the second device configured to generate a signalon the Vbus wire; wherein the first device is configured to determine apower rating of the USB cable by receiving and processing the signalfrom the second device through the receptacle.
 2. The system asspecified in claim 1, wherein: the receptacle has an ID pin and a Vbuspin; and the first device is configured to detect a presence of acapacitor coupled between the ID pin and the Vbus pin as a function ofthe signal received from the second device.
 3. The system as specifiedin claim 1, wherein: the receptacle has an ID pin and a ground pin; andthe first device is configured to detect a presence of a capacitorcoupled between the ID pin and the ground pin as a function of thesignal received from the second device.
 4. The system as specified inclaim 1, wherein the first device is configured to deliver power to theUSB cable while simultaneously detecting a presence of the signal fromthe second device.
 5. The system as specified in claim 1, wherein thefirst device is configured to adjustably establish the power deliveredby the first device to the USB cable as a function of a presence of thesignal and not a content of the signal.
 6. The system as specified inclaim 1, wherein the first device is configured to establish the amountof power delivered by the first device to the USB cable as a function ofthe determined power rating of the USB cable.
 7. The system as specifiedin claim 1, wherein the signal comprises one of: a GoodCRC controlmessage, a Ping control message, a HardReset packet, a Get_sink_capcontrol message, a Get_source_cap control message, a BitStream message,and a Capabilities message.
 8. The system as specified in claim 1,wherein the second device is configured to limit the amount of powerdelivered by the first device to the USB cable as a function of thedetermined power rating of the USB cable.
 9. A method of determining apower rating of a Universal Serial Bus (USB) cable extending between afirst device coupled to a near end of the USB cable and a second devicecoupled to a far end of the USB cable, the method comprising the stepsof: receiving a signal from the second device at the first device; anddetermining the power rating of the USB cable using the signal.
 10. Themethod as specified in claim 9, wherein: the first device comprises areceptacle having an ID pin and a Vbus pin; and determining the powerrating of the USB cable comprises detecting a presence of a capacitorcoupled between the ID pin and the Vbus pin as a function of the signalreceived from the second device.
 11. The method as specified in claim 9,wherein: the first device comprises a receptacle having an ID pin and aground pin; and determining the power rating of the USB cable comprisesdetecting a presence of a capacitor coupled between the ID pin and theground pin as a function of the signal received from the second device.12. The method as specified in claim 9, wherein the first devicedelivers power to the USB cable while simultaneously detecting apresence of the signal from the second device.
 13. The method asspecified in claim 9, wherein the first device adjustably establishesthe power delivered by the first device to the USB cable as a functionof a presence of the signal and not a content of the signal.
 14. Themethod as specified in claim 9, wherein the signal comprises one of: aGoodCRC control message, a HardReset packet, a Get_sink_cap controlmessage, a Get_source_cap control message, a BitStream message, a Pingmessage, and a Capabilities message.
 15. The method as specified inclaim 9, wherein the first device adjusts the power it delivers to theUSB cable as a function of the determined power rating.
 16. The methodas specified in claim 9, wherein the second device limits the powerdelivered to the USB cable by the first device as a function of thedetermined power rating.
 17. An apparatus comprising: a controllerconfigured to receive a signal sent to a first device coupled at a nearend of a Universal Serial Bus (USB) cable from a second device coupledat a far end of the USB cable; the controller configured to determine apower rating of the USB cable based on the received signal.
 18. Theapparatus as specified in claim 17, wherein the controller is configuredto adjust the amount of power delivered by the first device to the USBcable as a function of the determined power rating
 19. The apparatus asspecified in claim 17, wherein the controller is configured to limit theamount of power delivered by the second device to the USB cable as afunction of the determined power rating.
 20. The apparatus as specifiedin claim 17, wherein: the first device comprises a receptacle having anID pin and a Vbus pin; and the controller is configured to determine thepower rating of the USB cable by detecting a presence of a capacitorcoupled between the ID pin and the Vbus pin as a function of the signalreceived from the second device.
 21. The apparatus as specified in claim17, wherein: the first device comprises a receptacle having an ID pinand a ground pin; and the controller is configured to determine thepower rating of the USB cable by detecting a presence of a capacitorcoupled between the ID pin and the ground pin as a function of thesignal received from the second device.
 22. The apparatus as specifiedin claim 17, wherein the controller is configured to adjustablyestablish the power delivered by the first device to the USB cable as afunction of a presence of the signal and not a content of the signal.23. The apparatus as specified in claim 17, wherein the signal comprisesone of: a GoodCRC control message, a HardReset packet, a Get_sink_capcontrol message, a Get_source_cap control message, a BitStream message,a Ping control message, and a Capabilities message.